The present invention relates generally to portal imaging in radiotherapy systems and more particularly to improving image quality when providing portal images in such systems.
Radiation-emitting, devices are generally known and used for radiation therapy in the treatment of patients, for example. Typically, a radiation therapy device includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high-energy radiation beam for therapy. This high radiation beam can be an electron radiation or photon (X-ray) beam. During, treatment, the radiation beam is provided on one zone of a patient lying in the isocenter of gantry rotation.
A feature of radiation therapy involves portal images, which are commonly used in radiation therapy to verify and record the patient tumor location. Portal images, i.e., images of the port through the patient through which radiation emerges, include manual (film) and electronic images (EPI) taken before or after the treatment. Electronic portal images (EPI), when taken before the treatment, give the therapist the opportunity of correcting for minor patient positioning errors. Further, EPI allows therapists to take images remotely without going inside the treatment room.
In a typical portal imaging scheme, anti-scatter-grids (placed behind the patient and in front of the film) are used. The grids consist of small lead objects. The lead objects let pass most of the direct radiation from the x-ray source, but they block most of the scattered radiation that is travelling in other directions. In radiotherapy, the penetration of the used high-energy x-ray is so much higher that the anti-scatter grids are only applicable in diagnostics.
In the electronic portal imaging, x-rays are used to see through the body of the patient (for example, to decide if a bone is broken or not). The x-ray source is placed on one side of the patient and a film is exposed on the other side. Inside the patient radiation is attenuated (absorbed and scattered). The radiation is attenuated more by bone than by soft tissue. Therefore, on the film x-ray intensity is less behind a bone. (The aim is to have a big difference in intensity behind bone and soft tissue to have good contrast in the picture.) Scattered radiation reduces the contrast in the images. Current technology using electronic imagers generally provides poor quality images because the contrast is so small, which limits the ability to have electronic images replace film images for the portal radiation field.
Accordingly, what is needed is a method and system for improving the quality of portal images. The present invention addresses such a need.
A method and system for providing a portal image of an object utilizing a radiotheraphy system is disclosed. The radiotherapy system includes a system for providing at least one opening over the object. The at least one opening allows radiation therethrough. The method and system comprises acquiring a plurality of images through the at least one opening while moving the at least one opening over the object. Each of the plurality of images includes regions within the opening and regions outside the opening. The method and system includes obtaining a resulting portal image from the plurality of images. Radiation scattered to regions outside the opening is minimized in the resulting portal image to improve image quality.
A method and system in accordance with the present invention utilizes a virtual grid to provide a resulting portal image with more contrast. The virtual grid is provided through an opening or a slit. The slit is preferably provided by a plate/jaw system in the radiotherapy system. The plurality of images is provided via the use of software controlling the slit which causes a plurality of images to be provided over an object while the slit is moving over the object. Thereafter, when composing the resulting image, only the regions in the images within the moving slit area are used to compose the result image. The radiation scattered to regions outside the slit do not contribute to the resulting image. A considerable improvement in image quality is thereby provided.